1. Field of the Invention
The present invention relates to a method of identifying a semiconductor wafer during fabrication processes by irradiating infrared rays onto a back side of the wafer and receiving the reflected rays from a code pattern formed on a front surface thereof.
2. Description of the Related Art
In one known method of identifying a semiconductor wafer a code pattern is formed on a front surface of a wafer and light is irradiated on a front side thereof and the reflected light from the code pattern is received on the front side. By this conventional method, when a plurality of layers is deposited on the code pattern, it becomes difficult to discriminate the code signal from the reflected light.
In mass production of semiconductor devices of a specific kind, a method of lot production control is generally applied, wherein a plurality of wafers is subjected to a collective production control using the same lot number. In this case, there is no need for identifying each wafer, and a lot number is given to a carrier or container carrying a plurality of wafers.
However, when producing a small quantity of integrated circuits such as ASIC (Application Specific Integrated Circuits) or gate array integrated circuits, each wafer must be identified during the whole fabrication process.
In the known method of identifying each wafer, a code pattern, such as numeral or character marks, or preferably a bar code pattern, is formed by etching a silicon surface utilizing photolithographic technology or inscribing the surface with a laser beam.
A bar code pattern is usually formed in a peripheral region near an orientation flat edge of a silicon wafer. FIG. 1 shows schematically a cross-section taken along a line crossing the bar code pattern. The bar code pattern 22 is formed on a surface 21a of a silicon wafer 21 by inscribing and roughing the silicon surface with a laser beam. A light source 23 is provided on the front side of the wafer and irradiates a light beam 24 onto the bar code pattern. A reflected light beam 25 is received by a photodetector 26. By sweeping the light beam 24 over the bar code pattern 22, the bar code pattern is decoded and the wafer is identified.
The principle of the above-described method is relatively simple. The reflection from an original surface 21a of the wafer, which is not inscribed by the laser beam, is regular and stronger in a direction towards the photodetector 26, which is aligned in a direction according to the law of reflection. However, the reflection from the bar code pattern 22 is irregular and weak. When the output signal from the photodetector is subjected to a slice circuit, the circuit can selectively output a pulse signal corresponding to the bar code pattern.
The above method creates some problems which need to be solved. For example, the surface area where the code pattern is formed is subjected to wafer processes many times simultaneously with fabrication processes for the other remaining wafer area where active elements are formed. The processes include a depositing process of insulating or metal layers, chemical or physical etching processes, heating processes, etc. Also, when the difference in strength between reflected light from the code pattern and that from the wafer surface becomes smaller due to the deposited layers, a signal of the code pattern is difficult to discriminate.
In an extreme case, such as when a metal layer is deposited on an area of the code pattern, identification becomes impossible without removing the metal layer. FIG. 2 illustrates schematically a cross-section of the wafer 21, on which an insulating layer 31, a metal layer 32 of aluminum alloy, and a protective layer 34 are deposited on an area of the code pattern 22. A window 33 is formed in the metal layer 32 in order to allow the incident light beam 24 to impinge upon the surface of the wafer 21. Even if the window is formed, the intermediate layers (such as 31 and 34) disturb transmission of the incident and reflected light beams, resulting in a decrease of contrast between the reflected light from the code pattern and that from the wafer surface.
In order to solve the above problem, a method for identifying a wafer has been proposed in which a bar code pattern is formed in a metal layer, infrared rays are irradiated onto one side of the wafer, and transmitted rays through the wafer and the bar code pattern are received on the other side thereof. The method is described in U.S. patent application Ser. No. 051,398 on May 19, 1987 by T. Kiriseko et al. and T. Kiriseko, the later being the inventor of the present application.
Another method for identifying a wafer which attempts to avoid the above-described problem, is one in which a bar code pattern is formed on a bottom surface of the wafer and both a light source and a detector for receiving the reflected light are placed on the bottom side of the wafer, thus avoiding the effects of deposited layers on the front surface of the wafer. U.S. Pat. No. 4,010,355 to K. E. Roehrman et al. describes this type of method.
The method which involves forming the code pattern on the bottom surface of the wafer has some problems. One problem is that, when the code pattern is formed by inscribing the wafer surface by a laser beam and evaporating silicon material, silicon material is likely to build up on a peripheral portion of the pattern, thus deteriorating the flatness of the bottom surface. This in turn creates a defocusing problem in the photolithographic process for the front surface. Another problem is that when the bottom surface of the wafer is subjected to a grinding process before it is separated into chips for sealing into a package, the code pattern is erased.